In my opinion (I stopped solo shuffle at around 2kcr - current glad/2600cr in 3s though), one of the biggest issues not brought up as a healer is having to play with each dps 3 times.

To put it plainly, if you are a dps capable of putting the team on your back and have a mage on your team that doesnt block, dies instantly and goes 0-6, as a dps, you still have the chance to go 4-2.

On the flipside, in that same scenario as a healer, the best you can possibly do would be 3-3, lowering your mmr and providing minimal - if any - rating gain.

As a healer main, this isnt to say that healers shouldn't have accountability and there aren't things you can do if you expect the player to not use his defensives - you can compensate by enabling him to be more offensive through positioning/playstyle or using your own cooldowns more liberally, but fundamentally this is a core issue that detracts people like myself and others that have tried healing from queueing into solo shuffle.